The Second Doctor talking to Victoria in Tomb of the Cybermen
Susan leaving
The 7th doctor talking to Davros
The 6th doctor talking about how corrupt the Time Lords are
The 11th doctor in the Rings of Akhaten

Plenty of Eccleston moments, pretty much everything from 'Rose', perfectly re-establishing the Doctor, facing the lone Dalek in 'Dalek' ("I watched it happen. I made it happen!"), the speech at the end of Bad Wolf, the hologram speech, all amazing.

The Doctor
Oh big big mistake. Really huge. Didn't anyone ever tell you? There's one thing you never put in a trap—if you're smart, if you value your continued existence, if you have any plans about seeing tomorrow—there's one thing you never ever put in a trap.

The Second Doctor talking to Victoria in Tomb of the Cybermen
Susan leaving
The 7th doctor talking to Davros
The 6th doctor talking about how corrupt the Time Lords are
The 11th doctor in the Rings of Akhaten

Yes the 2nd dr and Victoria is a wonderful moving scene. An example of how universal and human Who can be

"Homo sapiens. What an inventive, invincible species. It's only a few million years since they crawled up out of the mud and learned to walk. Puny, defenceless bipeds. They've survived flood, famine and plague. They've survived cosmic wars and holocausts. And now, here they are, out among the stars, waiting to begin a new life. Ready to outsit eternity. They're indomitable. Indomitable."

Tom Baker really excels with Douglas Adams' scripts, clearly relishing Adams' shared love of the English language, and Adams seems to be able to write the best lines for Tom to say. And Tom's wonderfully rich voice is always a joy to listen to, especially when he's so clearly having fun.

One of my favourites is from City Of Death, and is made all the better by the cheerily offhand way that Tom comes out with - "I say; what a wonderful butler, he's so violent!"

"There are worlds out there where the sky is burning, and the sea's asleep, and the rivers dream. People made of smoke, and cities made of song. Somewhere there's danger, somewhere there’s injustice, and somewhere else the tea's getting cold. Come on, Ace — we’ve got work to do!"

I thought the Seventh Doctor was brilliant and this is a lovely line to end Classic Doctor Who.

Davros's megalomaniac speech in Genesis of the Daleks, and the way the voice treatment becomes more and more Dalek-like as his fervour raises.

"Yes, yes, to hold in my hand a capsule that contained such power... To know that life and death on such a scale was my choice... To know that the tiny pressure of my thumb, enough to break the glass, would end everything! Yes, I would do it! That power would set me up above the gods, and through the Daleks I shall have that power!"

Doctor 1 - "We are not of this race, we are not of this world."
Doctor 2 - "There are some corners of this Universe that have bred the most terrible things."
Doctor 3 - "Tears Sarah Jane? No, don't cry. While there's life there's...
Doctor 4 - "If someone you knew from the future pointed out a child to you and told you that child would grow up to be totally evil. A ruthless dictator who would destroy millions of lives. Could you then kill that child?"
Doctor 5 - "Don't look. You must not look. The Mara is feeding off your fear and finding it easy! You must not believe what you see. The Great Crystal is absorbing the belief and creating the Mara!"
Doctor 6 - "Madam. This revelation should halt this trial immediately. Surely even Gallifreyan law must acknowledge that the same person cannot be both prosecutor and defendant!"
Doctor 7 - "We hope to return to the North way., carrying home all the treasures from the Silk Lands to the East. But a dark curse follows our dragon ship."
Doctor 8 - "Cybermen. They were humanoid creatures much like you once. But their civilisation had a penchant for cybernetically improving upon the weaknesses of the flesh."
Doctor 9 - "If you can't kill, then what are you good for, Dalek? What's the point of you? You're nothing! What the hell are you here for?
Doctor 10 - "I'm the Doctor. I'm a Timelord. I'm from the planet Gallifrey in the constellation of Kasterborous. I'm 903 years old and I'm the man that's going to save your lives and all 6 billion people on the planet below. You got a problem with that?"
Doctor 11 - "But they keep coming back! Don't you see? Everytime I negotiate I try to understand. Well not today. No! Today I honour the victims first. His! The Master's! The Daleks! All the people who died because of my mercy!"

I think Julian Glover both in The Crusade and City Of Death had some of the best well written and wittiest lines ever. Both stories contain for me great examples of both but this is just my personal opinion.

Aside from this, I think Colin Baker had a great selection of dialogue. Some of his more emotional scenes such as his sadness and grief at Azmael's and then Peri's death during his time as The Doctor were delivered with an emotion that was unusual for the Classic Series Doctors.

For me, however, it was his big speech in Part 13 of TOATL where he really hits out at Time Lord Corruption and says all the anniversaries he's met are still in the Nursery compared to the Time Lords that stands out. Great stuff.

Also liked Hartnell's 'We are not from this world.' line from the very first episode.